1. Field of the Invention
This invention relates to zoom objectives and, more particularly, to zoom objectives of telephoto design with the range including long focal lengths.
2. Description of the Prior Art
In the past, zoom objectives of the type constructed from two parts, from front to rear, the varifocal part consisting of three lens groups of positive, negative and positive powers respectively, and the fixed parts consisting of two lens groups have become well known. Examples of such zoom objectives are shown, in U.S. Pat. Nos. 4,189,213, 4,192,577 and 4,223,981. The three lens groups constituting the varifocal part take the form of a 1st group counting from the front having the focusing function, a 2nd group having the magnification changing function and a 3rd group having the image shift compensating function. The fixed part has the function of focusing the object image bearing light beam from the varifocal part at a predetermined point of position. It is noted that there is another method of compensating for the image shift by using the 1st or focusing group in stead of the 3rd group.
In general, in the above-described type zoom lens design, in order to achieve greater compactness of the zoom objective by reducing the physical length and the diameter of the front lens members, it is advantageous to increase the refractive power of the 2nd group with the decrease in the total movement of the 2nd group. This contributes to a shortening of the total length of the entire lens system another consideration involved constructing the fixed part in the form of a telephoto type lens system, thus shortening the physical length of the fixed part. However, the increase of the refractive power of the 2nd group simultaneously requires that the refractive powers of the 1st and 3rd groups be increased with the sacrifice of good aberration correction, as the range of variation with zooming of the spherical aberration is increased, and gool stability of image aberration correction over the focusing range becomes difficult to achieve. This is because the load of refractive power on each of the lens surfaces in the 2nd group becomes heavy, and the resultant lens surfaces produces various aberrations which cannot be balanced out in the varifocal part.
On the other hand, when the refractive power of the 2nd group is weakened, the telephoto ratio of the fixed part must be decreased, or otherwise the advance in the compactness of the zoom objective would be failed. As a result, the Petzval sum amounts to a large negative value, and it becomes difficult to achieve good correction of aberrations.
It is preferred from the aberration correction standpoint that the 4th and 5th groups in the fixed part each be constructed with the inclusion of a positive lens and a negative lens. For the small telephoto ratio, however, the refractive power of the 4th group has to be strengthened. The introduction of the negative lens into the 4th group leads to a corresponding increase of the power of the positive lens. Therefore, the radius of curvature of the surfaces of the positive lens must be made smaller which in turn causes production of higher order aberrations and particularly spherical aberration.
The spherical aberrations of higher order can be usually corrected by the negative lens of the 5th group, but such correction of the higher order aberrations tends to largely affect the other various aberrations. Thus, general good correction of all aberrations becomes very difficult to achieve.
Further, in the lens making art, a problem arises wherein the tolerances within which a complete objective is assembled from its constituent elements and particularly the optical decentering tolerance become severer.